This invention relates to a limit switch which is used in a variety of manufacturing devices or industrial robots. A conventional limit switch of this type is shown in FIG. 24.
In FIG. 24, reference numeral 1 designates a housing which is made up of a switch housing body 3 incorporating a switch 2, and a cover 4 closing the side opening 3a of the switch housing body 3. A head housing 5 is detachably engaged with the upper end opening 3b of the switch housing body 3 with a plurality of bolts (not shown).
The head housing 5 has a shaft hole 6, in which a rotary shaft 9 is rotatably fitted with a collar 7 and an O-ring 8 so that one end portion 9a of the rotary shaft 9 extends outside the head housing 5. An actuator 10 is mounted on the one end portion 9a of the rotary shaft 9 and fixed with a screw 11. The actuator 10 includes a shaft member 13, on which a roller 12 is mounted.
The axis of the rotary shaft 9 is perpendicular to the axis of the operating rod 22 of the switch 2. A flat cam 9c is formed on the other end portion 9b of the rotary shaft 9 so that it is located on the axis of the operating rod 22.
A bottomed-cylinder-shaped spring receiver 14 is placed on the cam 9c. A return spring 15, which is a coiled spring, is set between the spring receiver 14 and the inner surface of the head housing 5, thus providing a rotational return force to the rotary shaft 9.
On the other hand, an operating plunger 16 is interposed between the cam 9c and the operating rod 22 of the switch 2. The operating plunger 16 comprises a cylindrical plunger body 17, an auxiliary plunger 18 built in the plunger body 17, and a buffer spring 19 interposed between the plunger body and the auxiliary plunger. The operating plunger 16 is axially movably inserted into the upper end opening 3b of the switch housing body 3. A depressing pin 20 for depressing the operating rod 22 is secured to the end of the auxiliary plunger 18.
The switch housing body 3 has a lower end opening 3c, which is closed with a connector (not shown) to which electrical cables are connected.
The limit switch thus constructed operates as follows: when an object such as a workpiece under test abuts against the roller 12, the actuator 10 is turned about the rotary shaft 9 against the elastic force of the return spring. As the actuator 10 is turned in this way, the cam 9c is turned to push the operating plunger, so that the switch 2 is operated.
When the cam 9c is turned in the above-described manner, the spring receiver 14 is displaced to compress the return spring 15. When released, the actuator is returned to the original position by the elastic force of the return spring 15 thus compressed.
In order to operate the switch 2, it is necessary to convert the rotational motion of the actuator 10 into the linear motion of the operating plunger 16. Therefore, the motion converting mechanism is intricate, and accordingly it needs a relatively large number of components, with a result that the limit switch is unavoidably bulky. Furthermore, the limit switch has a relatively short service life because it has a number of sliding parts and suffers from an unreasonable amount of stress.
On the other hand, it is essential to hermetically seal the internal chamber of the switch housing body 3 to protect the switch 2 from damage. To achieve this goal, it is necessary to set a cylindrical elastic seal member 21 in the upper end opening 3b to make the internal chamber 1a airtight.
However, it is rather difficult to maintain an airtight internal chamber for the following reason: In order to permit the reciprocation of the operating plunger 10, it is necessary for the internal chamber 1a of the switch housing 1 to change in volume and in pressure at all times; that is, it is necessary for the internal chamber 1a to breathe through the elastic seal member 21 to some extent. Thus, it is difficult to maintain an airtight internal chamber 1a. In addition, the breathing operation of the internal chamber 1a adversely affects the returning operation of the operating plunger, which results in a delay in the switching operation of the switch 2.